This invention pertains to austenitic stainless steels that contain a significant portion of delta ferrite, that will be exposed to elevated temperatures in service and that will be used in machine or equipment parts or members requiring stable dimensions. In this disclosure, the term "duplex" is used to describe stainless steels containing a dual microstructure of austenite plus delta ferrite. Specifically, the invention consists of a unique heat treating process by which the dimensions of such duplex stainless steels can be stabilized for service at elevated temperatures.
Ferrite-containing austenitic stainless steel castings are frequently specified for machine and structural elements because of their many desirable characteristics, such as:
(a) High yield strength
(b) Good resistance to hot tearing on casting
(c) Good resistance to hot cracking during welding
(d) Good resistance to stress-corrosion cracking.
They have one disadvantage--their metallurgical instability at elevated temperatures.
The invention is applicable to austenitic stainless wrought alloys, weld deposits or castings, as long as they contain delta ferrite. However, the invention is particularly useful in the treatment of castings because we have found that the high level of delta ferrite typically present in many commercial grades of stainless castings, for example 8 to 18%, can lead to large dimensional changes during exposure to elevated temperatures. As will later be described, a major portion of the shrinkage is caused by transformation of delta ferrite (.delta.) to austenite (.gamma.) plus the brittle sigma phase (.sigma.).
It is, therefore, an object of this invention to treat materials so as to stabilize their dimensions against further change during long service exposure to elevated temperature, without seriously impairing the ductility of the material through precipitation of large brittle particles.
It is also an object of this invention to provide for relief of residual stress during the course of the dimensional stabilization treatment and to accomplish both of these objectives in a reasonable and economic time period. Since the process to be described involves slow cooling through the carbide precipitation temperature range, the invention is preferably practiced on members that will not be exposed to corrosive environments. Alloys treated according to our invention perform satisfactorily in liquid sodium.